Retain length of handles when modifying curves and length of path when moving anchor points.
I'd love an option to retain the length of a handle for an anchor point. It would be nice to keep the curvature equal on both sides of an anchor point. There are many times I would have liked this option to create symmetrical graphics.
Similarly, I'd also like the option to retain the length of a path when moving an anchor point. Sometimes I use lines/paths as arms of characters. If I need to change the position of an arm there ends up being a lot of guess work modifying the path.
Stephen, there is a way to modify both handles at once, actually. Select a point and use Scale tool (S key) to scale both handles at once.
There is another method, to hold Alt with Pen tool and then dragging a new handles from any point, but you probably know of it.
Can you demonstrate some cases that need more that this?
(Yeah, I know a lot of time passed since your request)
As for the second thing, keeping a length constant — interesting idea. Not sure if it suites the Direct selection tool well enough, but the Reshape can benefit from it for sure.